Patent Document 1 has described that an evaluation value is calculated in accordance with a change of imbalance in concentration of lithium ions in an electrolytic solution, and the output from the secondary battery is controlled on the basis of the evaluation value. The evaluation value is a value for evaluating the deterioration of the secondary battery.
When the secondary battery is discharged, the lithium ions in the electrolytic solution move from a negative electrode toward a positive electrode to produce the imbalance in the concentration of the lithium ions in the electrolytic solution. Assuming that the imbalance in the lithium ion concentration is a factor in deterioration of the secondary battery, the evaluation value is calculated in accordance with the imbalance in the lithium ion concentration between electrodes in Patent Document 1.